The processing of printed circuit boards (PCB or board) may include a variety of steps, such as screening, placement, solder reflow, auto-insertion, wave soldering, press fitting, conformal coating, etc. Screening is the process of accurately applying solder paste to the areas of the PCB that will be receiving components. Screening is typically the first step in a surface mount process. Placement is the second step in the surface mount process. During the placement step, surface mount components are mechanically placed onto the pre-screened areas of the PCB. The final step of a surface mount process is ordinarily solder reflow, wherein the PCB temperature is elevated to a level sufficient to liquefy the solder paste applied during screening (typically at temperatures of about 300.degree. C. or higher). Reflow of the solder forms the electrical communication or bonds between the surface mount components and the PCB.
During auto-insertion processing of the PCB, through-hole components are mechanically inserted into the PCB. The wave soldering process forms the electrical communication or bonds between through-hole components and the PCB. During wave soldering, the top and bottom surfaces of the PCB are exposed to relatively high temperatures (typically at temperatures greater than about 300.degree. C.). The PCB travels over a molten bath of solder as the PCB is heated from both the top and bottom. The heat causes the molten solder to wick into the through-holes, forming the electric bond between the through-hole components and the PCB.
Press fit or compliant pin processing involves alignment of connector leads with barrels in the PCB, as force is applied to the board to fully seat the component. Press fitting establishes and/or ensures that there is an electrical connection between the PCB and the component. A conformal coating is then applied to the PCB Application of a conformal coating to the PCB protects the board from future, "severe" environments.
Each of the above processes may be more precisely performed utilizing a fixture to support and secure the PCB during the process. The prior art has attempted to accommodate such needs. For example, U.S. Pat. No. 3,395,439 to Palesi discloses an apparatus for securing a PCB for attachment of components thereto. The Palesi apparatus includes a frame with a pair of opposing side members spaced apart by end rods. A groove within each side member receives an edge of the PCB to hold the board during a dip soldering process. Securing of the board in the frame is accomplished by placement of a solid top plate attached to the frame at four corners, by thumbscrews. Because the top of the Palesi fixture is a solid plate, the top surface of the PCB is not accessible for performing PCB processing steps other than dip soldering. Further, the solid top plate acts as a heat sink, disturbing processes requiring that the heat be evenly distributed to the PCB, such as during the wave soldering process.
Additionally, the electronics industry is requiring thinner and thinner PCBs. Accordingly, uniform application of pressure to the PCB secured in a fixture for processing is critical to ensure precise and accurate PCB processing during processes such as wave soldering. Separate manipulation of connectors, such as the thumbscrews of the Palesi device, result in non-uniform pressure on the board causing creation of a moment in the board.
Existing fixtures are devices such as Palesi, wherein the devices have a bottom and a top piece. The top piece requires an operator to manipulate multiple connections at multiple locations to secure the PCB in the fixture. Such fixtures result in uneven pressure being applied to the PCB, creating undesirable moments in the boards that result in inaccurate processing of the boards. Non-concurrent manipulation of multiple fixture connectors on the fixture results in the creation of such moments in the PCB. Specifically, during such manipulation of the first connector, one portion of the board has pressure exerted thereon by the connector while other parts of the board experience little or no pressure. This creates an undesirable moment in the board. In addition to requiring individual operation, the fixture connectors of the prior art devices are such that, even if the connectors were to be simultaneously operable, equal pressure is not applied to the board due to the unequal pressure applied by each of the connectors.
Alternative prior art fixtures are devices having a frame and over arm bar, wherein the over arm bar is permanently attached to one side of the frame. The prior art over arm bars pivot about the permanently attached end. Such pivot action creates undesirable pressure on the boards. That is, the pivoting bar first contacts the PCB upper surface at an angle, exerting pressure on the board at an angle. The angular pressure produces an undesirable moment in the board. Although some prior art over-arm bar fixtures have attempted to reduce the moment created in the PCB by adding a cam-type mechanism to the pivoting bar, unacceptable moments are still created, especially in the thinner PCBs.
Concern regarding operator health has also brought about recent expectations or requirements for such PCB fixtures. Repetitive operator movement is to be limited and should be "ergonomically correct" motions. Prior art PCB fixtures require excessive, often ergonomically incorrect, operator manipulation to open and close the fixtures.
Additionally, many PCB processes require access to both the top and bottom surfaces of a PCB at the same time. Prior art PCB fixtures do not allow simultaneous access to both surfaces of the PCB or severally limit exposure of one or both surfaces of the board. Moreover, because both sides of the PCB may be heated during a particular process, such as wave soldering, the fixture cannot act as an excessive heat sink. That is, for particular PCB processes, it is necessary that both sides of the PCB be exposed to essentially the same temperature, to effect process steps such as solder wicking.
Accordingly, there is a need for PCB fixtures that (1) do not apply non-uniform pressure to the board resulting in creation of a moment in the board, (2) allow exposure of both top and bottom PCB surfaces during processing, (3) require minimal operator movement and minimize or eliminate need for ergonomically incorrect, repetitive operator motion, (4) do not act as undesirable heat sinks, and/or (5) provides one or more hold down devices for securing or placing components on a PCB.